Encased variable resistance device



Nov. 24, 1959 H. M. SCHENDEL 2,914,744

ENCASED VARIABLE RESISTANCE DEVICE Filed Feb. 27, 1958 INVENTOR. HERBERTM. SCHENDEL BY ,A/

ENCASED VARIABLE RESISTANCE DEVICE Herbert M. Schendel, Santa Ana,Calif., assignor to Beckman Instruments, Inc., a corporation ofCalifornia Application February 27, 1958, Serial No. 717,907

7 Claims. (Cl. 338164) This invention relates to variable resistancedevices and, in particular, to variable resistance devices which areencased and sealed from the atmosphere.

One of the principal limiting factors in the design of precisionvariable resistance devices such as potentiometers is environmentalsensitivity. A typical, non-enclosed, variable resistance device orpotentiometer comprises a sliding contact which is adjusted to desiredpositions on a resistance element by means of a rotatable shaft. Ofcourse, whenever the potentiometer shaft is turned, wear particles areproduced from the shaft bearings, the sliding contact and from any otherparts that move in conjunction with the shaft. These wear particles findtheir way between various moving parts in the resistance device, causinga variety of problems. Shaft bearing failure, electrical noise generatedby the sliding contact and dielectric failure are typical examples.Moreover, ambient moisture produces galvanic action and other types ofchemical corrosion, along with distortion of component insulatingmaterials as a result of absorption of moisture. Wear products,moisture, electrical potential and heat generated from the passage ofelectrical current create a combination of electro-chemical effects thatcan and do produce costly deterioration in variable resistance devices.

It is the principal object of present invention to provide a variableresistance device which is relatively insensitive to environmentalconditions.

It is a further object of the present invention to provide a variableresistance device which has a greatly increased life expectancy.

It is yet another object of the invention to provide a variableresistance device in which movement of the sliding contact is damped toreduce substantially or eliminate bouncing and peening of the slidingcontact.

It is a still further object of the invention to provide a variableresistance device which has reduced electrical noise effects from thesliding contact and improved ability to withstand shock and vibration.

It is another object of the invention to provide a variable resistancedevice which is capable of dissipating substantially increased amountsof power under varied environmental conditions.

United States Patent According to one aspect of the present invention,these and other objects are realized by disposing a variable resistancedevice in a liquid-tight case and filling the case with a suitableinsulating medium such as oil. Terminal means extending through the casemake variable resistance values available exterially of the case.

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded asconstituting the present invention, it is believed that the inventionwill be better understood from the following description, taken inconnection with the accompanying drawing in which the single figure is aView, partially in section, of a variable resistance device constructedin accordance with the present invention.

Referring particularly now to the drawing, there is shown an encasedvariable resistance device according to the present invention whichcomprises a variable resistance device 1 disposed within a case 2.Device 1 may suitably be a variable resistance unit such as thosevariously described and claimed in A. O. Beckman U.S. Patent Nos.2,454,986 and 2,473,048, both of which are assigned to the assignee ofthe present invention. It is not necessary, however, that device 1 be amulti-turn variable resistance unit; instead, it may conveniently be asingle-turn variable resistor of a type well known to those skilled inthe art. From an economic point of view it is advantageous for device 1to be of a design which is available commercially and, in such event,the conventional lid or cover element should be removed for a purposewhich will appear more clearly in the following paragraph.

Variable resistance device 1 comprises a housing 3 of suitable plasticmaterial such as phenol formaldehyde. Disposed along the interiorperipheral surface of housing 3 in helical grooves accurately cuttherein, is a resistance winding 4 which is constructed of a pluralityof turns of wire 5 space wound in helical configuration upon aninsulated core 6 of material having relatively high heat conductivity.Conductive contact may be made to individual turns of wire 5 by means ofa slidable contact member 7. Contact member 7 is slidably supported upona rotor member 8 which may be turned through a shaft 9 that is journaledin a bearing 10, the mounting of which will be more fully describedhereinafter. tact member 7 is provided with a non-conductive guideelement (not shown) which extends into spaces 11 between adjacent turnsof helical winding 4 and guides member 7 along winding 4 as rotor 8 isturned. The upper and lower ends of resistance wire 5 are respectivelyconnected to terminals 12 and 13 which extend through and into housing 3to facilitate affixing the wire ends thereto by means of soldering orwelding. Contact member 7 is connected to a terminal 14 by means of aconventional slip ring (not shown).

As has been mentioned heretofore the principal objects of the presentinvention are realized by disposing variable resistance device 1 withinliquid-tight case 2. It is also stated above that variable resistancedevice 1 can conveniently comprise a variable resistance device orpotentiometer available commercially. In such event the conventionalcover or lid portion is removedso that the remainder of the device canbe attached to a lid member 15 as illustrated in the drawing. Resistancedevice 1 is attached to lid member 15 by means of screws 16 whichthreadably engage a split expansion ring 17 that, in turn, fits into aperipheral groove or slot in housing 3 as shown. Shaft 9, which isemployed to turn rotor member 8 from the exterior of case 2, is asmentioned above journaled in bearing 10, the outer race of which isdisposed in lid member 15 and retained therein with a split expansibleretainer ring 17. The shaft and rotor assembly is positioned andretained within lid member 15 by means of a split, dish-shaped springmember 19 that engages a slot 29 in shaft 9. O-rings 21 of a materialsuch as natural or synthetic rubber provide liquidtight seals forrotatable shaft 9.

In order that variable resistance values from device 1 may be availableexteriorily of case 2, terminals 12, 13 and 14 are respectivelyconnected to individual terminal means 22 through conductors 23, 24 and25. The conductors are preferably attached to terminals 12, 13 and 14 bymeans of solder in order to provide good electrical connection. Terminalmeans 22 are introduced through case 2 in insulating and liquid-tightfashion by means of hermetic seals 26 of a design well known to thoseskilled in the art. Conductors 23, 24 and 25 may Conbe protected andinsulated from each other and case 2 adjacent terminal means 22 withinsulating caps 27. Conductors 23, 24 and 25 are purposely provided withextra length as illustrated in the figure in order that the soldering orotherwise afiixing of conductors 23, 2 and 25 to terminals 12, 13 andlid and terminal means 22 may be accomplished while resistance device 1is positioned exteriorly of case 2 prior to permanent mounting in amanner which will be more fully described hereinafter. Short-circuitsbetween terminals 12, 113, 14 and case 2, as Well as other accidentalshort-circuits to the internal surface of case 2, are prevented bypositioning a thin sheet 28 of insulating material such as Mylar filmaround the inner periphery of case 2. Mylar film is a type of saturatedpolyester prepared from ethylene glycol and terepthalic acid and is soldcommercially by E. I. du Pont de Nemours & Co., Inc, Wilmington,Delaware.

For the purpose of providing an expansible volume within case 2 there isprovided a bellows 29 which comprises a collapsible member 3% composedof a resilient material such as natural or syntheticrubber. Member 30 issealed to the adjacent inner surface of case 2 in liquid-tight fashionby means of a washer member 31 which is provided with a threaded boss 32that extends through case 2 and is mounted thereon by means of a nut 33and washer 34. The volume 35 enclosed by resilient member 30communicates with the external atmosphere through a passage as in boss32 and washer 31. In order to prevent bellows 29 from collapsing whenfluid is inserted in case 2 in a manner to be described later, aresilient supporting member 37, preferably composed of natural orsynthetic sponge rubber, is positioned within resilient member 30 ofbellows 29. Communication of passage 36 with the volume enclosed byresilient member 30 is enhanced by removing a central portion 38 fromresilient member 37.

The encased variable resistance device of the invention may be assembledin the following manner. Bellows 29 and terminal means 22 withconductors 23, 24 and 25 attached thereto are respectively mounted incase 2 in fashion which will be obvious to those skilled in the art.Variable resistance device 1 is pre-assembled exteriorly of case 2 andcompletely tested in conventional manner. Either before or aftertesting, device it may be mounted upon lid member 15 by means of screws16 and bearing 10 as previously described. Conductors 23, 24 and 25 'arethen respectively attached exteriorly of case 2 to terminals 12, 13 and14 of resistance device 1. Following this step the assembly ofresistance device 1 and lid member 15 is positioned within case 2 whichis then roll-sealed into a groove 40 containing an 6 ring 41.

After variable resistance device 1 has been secured within case 2, theentire assembly is placed in a vacuum oven and baked for a suitablelength of time to remove moisture from the interior of the case. Accessto the interior of case 2 is provided through a passage 42 that isnormally sealed by a screw 43, which is threadably engaged with lidmember 15, and resilient washer 44, Of course, for the baking procedurescrew 43 and washer 44 are removed from lid member 15. After the bakingprocedure is completed, the interior of case 2 is filled with a suitableliquid such as an oil conventionally used as a dielectric in thetransformer industry. Such oils are usually hydrocarbon compounds with amineral oil base, or silicone oils. It should be noted that theresilient member 37 within bellows 29 prevents the bellows fromcollapsing when case 2 is filled with oil. After the filling of case 2is finished, screw 43 and washer 44 are properly positioned to closepassage 4-2 and completely seal the assembly.

During operation of the encased variable resistance device of theinvention it is desirable to have as much movement as possible of theliquid in case 2 in order that heat generated by passage of currentthrough resistance device 1 may be conducted therefrom to the case 2,in:

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eluding lid member 15, for dissipation into the atmosphere. To providepassage for convection currents of fluid within case 2 and resistancedevice 1, holes 45 (one of which is shown) may be drilled or formed inhousing 3 of device 1. The convection currents of liquid passing throughholes 45 not only provide means for carrying heat away from resistancedevice 1 but also provide means for removing wear particles from theinner surface of helical winding 4 and contact member 7.

While the invention has been described by reference to a particularembodiment thereof, it will be understood by those skilled in the artthat numerous changes may be made without departing from the invention.It is therefore intended in the appended claims to cover this and allsuch equivalent variation of application and structure as are within thetrue spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. In combination: a variable resistance device having terminals and arotatable shaft for adjusting the value of resistance appearing acrossat least two of said terminals, a liquid-tight case enclosing saidvariable resistance device and having a lid portion sealed therein, saidrotatable shaft being journaled in said lid portion and extendingexteriorly thereof to permit rotation of said shaft from outside saidcase, means for filling said case with an insulating liquid medium,expansible bellows means disposed within said ease to permit expansionand contraction of liquid medium within said case, terminal means sealedin said case adjacent said terminals of said variable resistance device,and means conductively interconnecting respectively said terminals andsaid terminal means to make variable resistance values availableexteriorly of said case.

2. A combination as in claim 1 in which said means for filling said casecomprises a removable sealing member disposed in a passage in said lidportion.

3. A combination as in claim 2 in which said means conductivelyinterconnecting respectively said terminals and said terminal meanscomprises conductors having lengths sufiicient to permit connecting endsof said conductors to said terminals on said variable resistance deviceprior to assembly of said device within said case.

4. In combination: a variable resistance device including a housing, aresistance element within said housing, a slidable contact adjustablealong said resistance element, terminals mounted in said housing andconnected respectively to said resistance element and said slidablecontact, and a rotatable shaft extending exteriorly of said housing foradjusting said slidable contact to provide variable resistance valuesacross said terminals; a liquid-tight case enclosing said variableresistance device and having a lid portion sealed therein, saidrotatable shaft being journaled in said lid portion and extendingexteriorly thereof to permit rotation of said shaft from outside saidcase; means for filling said case with an insulating medium, saidvariable resistance device being disposed in said case to permit flow ofliquid convection currents within said case and through said device;expansible belows means disposed within said case to permit expansionand contraction of liquid medium within said case; terminal means sealedin said case adjacent said terminals of said variable resistance device;and means conductively interconnecting respectively said terminals andsaid terminal means to make variable resistance values availableexteriorly of said case.

5. in combination: a variable resistance device including a housing, aresistance element within said housing, a slidable contact adjustablealong said resistance element, terminals mounted in said housing andconnected respectively to said resistance element and said slidablecontact, and a rotatable shaft extending exteriorly of said housing foradjusting said slidable contact to provide variable resistance valuesacross said terminals; a liquidtight case enclosing said variableresistance device and having a lid portion sealed therein, saidrotatable shaft being journaled in said lid portion and extendingexteriorly thereof to permit rotation of said shaft from outside saidcase; means for filling said case with an insulating medium, saidhousing having passages therein to permit flow of liquid convectioncurrents within said case and through said housing; expansible bellowsmeans disposed within said case to permit expansion and contraction ofliquid medium within said case; terminal means sealed in said caseadjacent said terminals of said variable resistance device; and meansconductively interconnecting respectively said terminals and saidterminal means to make variable resistance values available exteriorlyof said case.

6. In combination: a variable resistance device including a housing, aresistance element within said housing, a. slidable contact adjustablealong said resistance element, terminals mounted in said housing andconnected respectively to said resistance element and said slidablecontact, and a rotatable shaft extending exteriorly of said housing foradjusting said slidable contact to provide variable resistance valuesacross said terminals; a liquidtight case enclosing said variableresistance device and having a lid portion sealed therein, saidrotatable shaft being journaled in said lid portion and extendingexteriorly thereof to permit rotation of said shaft from outside saidcase; means for filling said case with an insulating medium, saidvariable resistance device being disposed in said case to permit flow ofliquid convection currents within said case and through said device;expansible bellows means disposed within said case to permit expansionand contraction of fluid medium within said case, said bellows meansincluding a collapsible member enclosing a resilient supporting member;terminal means sealed in said case adjacent said terminals of saidvariable resistance device; and means conductively interconnectingrespectively said terminals and said terminal means to make variableresistance values available exteriorly of said case.

7. In combination: a variable resistance device including a housing, aresistance element within said housing, a slidable contact adjustablealong said resistance element, terminals mounted in said housing andconnected respectively to said resistance element and said slidablecontact, and a rotatable shaft extending exteriorly of said housing foradjusting said slidable contact to provide variable resistance valuesacross said terminals; a liquidtight case enclosing said variableresistance device and having a lid portion sealed therein, saidrotatable shaft being journaled in said lid portion and extendingexteriorly thereof to permit rotation of said shaft from outside saidcase; means for filling said case with an insulating medium, saidvariable resistance device being disposed in said case to permit flow ofliquid convection currents within said case and through said device;expansible bellows means disposed within said case and communicatingthrough a passage in said case with external atmosphere to permitexpansion and contraction of fluid medium within said case, said bellowsmeans including a collapsible member enclosing a resilient supportingmember having a portion removed therefrom adjacent said passage in saidcase to enhance communication with external atmosphere; terminal meanssealed in said case adjacent said terminals of said variable resistancedevice; and means conductively interconnecting respectively saidterminals and said terminal means to make variable resistance valuesavailable exteriorly of said case.

References Cited in the file of this patent UNITED STATES PATENTS1,238,124 Frickey Aug. 28, 1917 1,389,105 Simmon Aug. 30, 1921 2,122,370Harrison et al. June 28, 1938 FOREIGN PATENTS 118,613 Great Britain June26, 1919 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No2,914,744 November 24, 1959 Herbert M. Schendel It is hereby certifiedthat error appears in the printed specification of the above numberedpatent requiring correction and that the said Letters Patent shouldreadas corrected below.

Column 4, line 39, for the claim reference numeral "2" read l line 61,for "below" read bellows Signed and sealed this 3rd day of May 1960.,

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner ofPatents v UNITEDv STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentN0 2,914,744 November 24, 1959 Herbert M. Schendel It is herebycertified that error appears in the printed specification of the abovenumbered patent requiring correction and that the said Letters Patentshould read as corrected below.

Column 4, line 39, for the claim reference numeral "2" read l line 61,for "belows" read em bellows Signed and sealed this 3rd day of May1960.,

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner ofPatents

